1. Field of the Invention
The present invention relates to a stencil printing machine in which a stencil sheet is perforated by a thermal head and a platen roller and then wound around a printing drum for stencil printing.
2. Description of the Related Art
FIG. 2 is a sectional view showing a part of a stencil printing machine where a stencil sheet is used.
The stencil sheet 100 in a roll is perforated by a thermal head 101 and a platen roller 102 of a perforating section while being pinched and conveyed therebetween. The stencil sheet is conveyed by conveying rollers 103 into a storing section 104 and stored therein. While a used stencil sheet wound around the printing drum 105 is discharged, the stencil sheet in a length to be used for one printing-operation is previously perforated and stored in the storing section 104. The stencil sheet 100 previously perforated is fed out by rollers 106 in a predetermined timing and conveyed to the printing drum 105. A leading end of the stencil sheet 100 is held on the printing drum 105 by a clamp plate 107, and the sheet is wound around the printing drum by rotating the printing drum. Then, the stencil sheet 100 is cut by a cutter 108 in the length for one printing-operation. Nest,a printing sheet is fed between the printing drum 105 and a pressing roller 109 in synchronization with rotation of the printing drum 105 and upward movement of the pressing roller 109. The printing sheet is printed while being pinched and conveyed between the printing drum 105 and the pressing roller 109.
The stencil sheet is composed of a thermoplastic synthetic resin film such as a plastic film, and a porous substrate laminated with the film. As the porous substrate, there can be mentioned porous thin paper such as Tengujo-type Japanese paper, porous sheet made from synthetic fibers, woven fabric, and non-woven fabric. The stencil sheet is normally conveyed by the perforating section in such a manner that the thermal head 101 contacts the thermoplastic synthetic resin film and the platen roller 102 contacts the porous substrate.
The stencil sheet is charged with static electricity while being pinched and perforated by the thermal head and the platen roller. That is, high nip pressure is exerted on the stencil sheet by the thermal head and the platen roller, so that the stencil sheet is conveyed in forceful contact with the thermal head and the platen roller during perforating. Therefore, the static electricity charge is inevitable.
Thus, a problem arises as shown in FIG. 2. Namely, the stencil sheet 100 sticks to a circumferential surface of the platen roller 102. Additionally, the stencil sheet 100 is distorted in a twisting way in the storing section 104 by the static electricity. If the stencil sheet 100 passes through the rollers 106 when it is conveyed to the printing drum 105, the stencil sheet 100 is folded. Therefore, the stencil sheet is wrinkled when being wound around the printing drum after being stretched. Further, a conveying path of the stencil sheet 100 is made of metal plates, and this causes another problem that the stencil sheet 100 sticks to the metal plates by the static electricity, so that the conveyance of the stencil sheet is intercepted.
The object of the present invention is to solve the problems of the static electricity charged on the stencil sheet in the stencil printing machine in which the stencil sheet is perforated by the thermal head and the platen roller.